There are currently known a wide variety of designs for transformers for signal, e.g. data and audio and similar applications. The design of a transformer is governed both by characteristics required of the transformer itself, as well as the production and assembly requirements necessary for each design.
For example, one known design available from COSMO and the subject of U.S. Pat. No. 4,716,394 has two separate bobbins for the two transformer coils, intended to provide high isolation between primary and secondary windings in power transformers. These two bobbins sub-assemblies are then mounted together in a U-shaped housing.
Another known design has a one piece two-section or three flanged bobbin. A central or intermediate flange separates the two sections of the bobbin. Such an arrangement can suffer from distortion during winding, can suffer from poor coupling between the windings and can provide inadequate creepage distances, to isolate the two coils.
Due to these problems and new standards in Europe and elsewhere concerning creepage and clearance distances at specified working voltages, the two bobbin configuration is often preferred.
However, many existing two bobbin designs suffer from a number of drawbacks or disadvantages. They lack performance requirements for transformers, as well as being ill suited to emerging manufacturing techniques, for manufacturing the transformer itself and for installing a transformer in or on a circuit board. This type of installation is becoming common.
For example, with the advent of Surface Mount Technology (SMT) and hybrid SMT through hole technology, reflow soldering has expanded rapidly. Several distinct challenges lie in applying this technology to a relatively large signal transformer. Different rates for thermal expansion of elements of the transformer (eg. laminations to thermoset plastics), high temperatures which may affect unprotected elements of the transformer (magnetic wire) and the relatively large mass of the transformer for mechanical stability to the circuit board, can all provide challenges for current manufacturing techniques. This is not provided for in many current designs.
For example, a resurgent technique for soldering, known as infrared reflow, is becoming common. In this soldering technique, energy is provided by infrared radiation. Various infrared sources are used. One system that is becoming common uses area or panel emitters providing medium to long wave infrared radiation. Heat is transferred by a combination of radiation, convection and conduction. It also heats the surrounding air by absorption, which supports heating of the parts and the printed circuit board (PCB). The band is also adjusted to the absorption coefficient of the PCB material, which heats the contact pads on the PCB and solder paste. The heating profile to which any individual part is subjected is determined by the component with the highest heat capacity. As a result, some parts may see excessive heat or excessive heating ramps, and mass differences alone can account for 50.degree. C. differences in temperature during a preheat ramp. While gradual heat ramps can be used to minimize this effect, it is nonetheless possible that some components may be subjected to excessive heating. Further, some equipment does not heat evenly across the width of a chamber in which the soldering process takes place. This can be caused by reflection or uneven transmission of radiation, or by the energy absorption of transfer conveyors. Consequently, a disadvantage of this technique is that it heats up the components significantly, and indeed can cause individual components to be subjected to excessive temperature profiles. For this reason, where this soldering technique is used, the individual components must be capable of accepting the necessary temperature profiles and must be capable of the necessary thermal expansion and contraction. This is not provided for in many current designs. Also, the design should be such as to shield relevant components from the infrared heat to the extent necessary.
The use of automated assembly techniques, such as pick and place at the circuit board level require that individual components be provided with elements that will accurately locate these components for through hole insertion or with respect to a pad placement on the circuit board. This is generally not provided for in placement transformer assemblies where the core of the transformer assembly has to be the conventional locating technique.
While known designs of transformers including side by side coils are suitable for power applications, they are unsuited to audio and other applications. For low frequency power applications (less than 100 Hz), one can accept a relatively low degree of efficiency. For high frequency applications, such as audio, data and other information signals, a greater degree of efficiency and coupling is required to maintain signal levels and signal to noise ratios and the like. Leakage inductance for high frequency response should also be minimized. This is accomplished in a concentric design.
For such high frequency applications, in particular signal transformers, it has been proposed to use two coils mounted on a common bobbin. In the past, these devices have been manufactured by placing the first winding on the bobbin and then placing an insulation barrier over the first winding which generally consists of an electrical grade insulating material such as tape. This tape is generally placed in the coil form such that the edges of the tape roll up the sides of the bobbin flange forming a pocket. The second winding is then wound in this pocket. Since the tape is generally very flexible and pliable there is no guarantee that the tape has not folded over on itself or has adequately covered the first winding and hence the insulation integrity may be suspect. For this reason, this configuration is not acceptable for safety requirements to the newer European regulations, which will likely be adopted in North America. There is therefore a need for a transformer design which will provide the coupling requirements for audio, data or other higher frequency signal applications, which will meet current safety regulations and which will be consistent and repeatable in the manufacturing process.
A further problem is encountered in the assembly of signal transformers having to operated with a DC bias being applied to the device. Conventional laminated devices would saturate and therefore cannot be used.
The technique commonly used is to place either an EE or EI ("E" and "I" indicating the shape of the laminations in known manner) configured laminate core in the device using an electrical insulating barrier of a specified thickness (gap) between either the EEs or EIs. This would then be held in place usually by tape around the outside of the laminations. The thickness of the gap may be varied depending on the winding characteristics and the electrical properties of the lamination. This assembly may optionally be held together by wrapping tape around the assembly which may also be a determining factor both in performance and further processing of the transformer. Conventional techniques lend themselves to additional reworking, poor handling and placement of the lamination especially the EIs and poor handling and placement of the gap material. This coupled with taping the assembly provides a difficult task in many known transformer assembly techniques, requiring significant manual dexterity.